Dyntopo: Re-symmetrize only part of mesh
Best bet is to do all your mirrored sculpting first,
then turn mirror off, and finalize asymmetrical details.  

For instance:  https://www.youtube.com/watch?v=TLb_CCoWn5o  

If you've already started sculpting without symmetry,
it's possible to copy your sculpture, then place a cube around the area.  

Use boolean modifier set to intersect on one sculpt &amp; cube,
Use boolean modifier set to difference on the other sculpt &amp; cube.

Continue sculpting on the area that only has symmetrical details.  

Merge the two halves with a boolean union once you're finished,
and smooth any seam with the sculpt tools.